osmotic pressure of bovine serum albumin in the presence of calcium chloride with low ionic strength

It is well known that the osmotic pressure of protein solutions deviates from ideal behavior at high concentrations. Our lab previously developed a free-solvent model for osmotic pressure that provides excellent prediction of the osmotic pressure for protein solutions at moderate ionic strength using only physically-realizable parameters. The model assumes that the hydrated proteins behave ideally in solution as the electrostatic interactions in the high ionic strength solutions are largely screened. In this study, we evaluate the predictability of the free-solvent model for protein solutions made up of CaCl2 solutions at low ionic strengths. In low ionic strength solutions, the Debye length can extend out to be on the order of the Stokes-Einstein radius of the protein; therefore electrostatic influences could induce protein-protein interactions. Using osmotic pressure data from solutions of bovine serum albumin (BSA) in 3 mM CaCl2 at pH 7.4, we estimated the parameters, hydration and ion-binding, of the free-solvent model using a non-linear least squares regression algorithm. The results generated an excellent fit to the data but produced a negative ion binding parameter, (-2.67 ± 0.241 mol CaCl2/mol BSA). Thus, the free-solvent model is highly sensitive to ion binding variations but results in an excellent fit to the highly-nonlinear monotonic data. These results suggest that the general concept of the free-solvent has validity, but that perhaps additional corrections of protein-protein or other interactions must be introduced to account for the observed osmotic pressure at low ionic strength solutions. Furthermore, HPLC results of BSA in 3 mM CaCl2 shows two peaks at 278 nm, suggesting that the BSA solution used has a monomer and a lower MW species; thus a two-protein model for this solution can also be considered. keywords: osmotic pressure, free-solvent model, bovine serum albumin, salt ion binding danielle nicole ornelas Department of Bioengineering danielle nicole ornelas is a graduating senior in bioengineering. she presented her research at the ucr symposium for the undergraduate research, scholarship, and creative activity and will also be presenting at the upcoming uc systemwide bioengineering conference at uc san diego and the biomedical Engineering society annual conference in seattle. danielle joined the biotransport and bioreaction Kinetics (b2K) group in may 2012 and has been involved in osmotic pressure research that allows her to understand the behavior of osmotic pressure of concentrated protein solutions. she worked closely with the loma linda Pediatric surgery department to help design and build an infant surgical simulator that facilitates the practice of duodenal atresia surgery. danielle is eager to pursue a m.s. in bioengineering, followed by further graduate work in the medical field. she is an active member of the health sciences Partnership mentor Program, works as a student success counselor for the bourns college of Engineering, and has dedicated seven years of her time volunteering at a local hospital. danielle gives special acknowledgement and thanks to noriko uka ozaki, devin mcbride, and dr. victor g. J. rodgers for their enduring support and guidance, and for helping to make her the researcher she is today. a copy of this paper in its entirety can be found online at www.ugr.ucr.edu in Volume VII. f A C u l T y M e n T o R Victor G. J. Rodgers Department of Bioengineering Overseeing the B2K Group (Biotransport and Bioreaction Kinetics), Professor Rodgers research interests combine experiemental work with mathematical analysis to address problems related to biotransport phenomena, bioreaction kinetics and thermodynamics in biomedical engineering and bioseparations. OSMOTIC PRESSURE OF BOVINE SERUM ALBUMIN IN THE PRESENCE OF CALCIUM CHLORIDE WITH LOW IONIC STRENGTH